Method and apparatus for assessing the ripeness or firmness of fruit and vegetables

ABSTRACT

Apparatus for testing fruit and vegetables to assess their ripeness includes an impactor device ( 28 ) comprising a bellows ( 30 ) which can be expanded and retracted by the application of pressurized air and vacuum via a support tube ( 32 ) for the bellows and which mounts an impactor ( 33 ) for tapping a fruit or vegetable item to be tested. The impactor has an internal slug movable relatively to the bellows ( 30 ) so that, when the bellows expands and stops upon its nose piece ( 36 ) contacting the surface of the item to be tested, the slug continues to move through the aperture ( 35 ) in the nose piece, under its own momentum, to tap the surface of the item. The slug incorporates a force transducer which, when the slug is tapped against the item, produces an electrical output signal in the form of a pulse corresponding to the reaction force and this pulse is processed to produce a signal indicative of the ripeness of the fruit.

FIELD OF THE INVENTION

The present invention relates to a method and apparatus for testingfruit and vegetables to assess their firmness or ripeness.

BACKGROUND OF THE INVENTION

Knowing the degree of firmness or ripeness of fruit or vegetables (inthe following description and claims referred to for convenience simplyas fruit) is a factor of considerable commercial importance as itenables importers and distributors, for example, to assess theshelf-life of the fruit and meet the requirements of supermarkets andother retail outlets in this regard. When picked, even fruit from thesame tree or plant is of different ripeness and any assessment made atthis time is unreliable. Thus, boxes of fruit picked at the same timecontain fruit with different degrees of ripeness. After picking, fruitis stored and transported under refrigerated conditions in order toprevent further ripening. Prior to supply to a retail outlet, theimporter or distributor removes the fruit from cold store and exposes itto a warm environment to ripen it. It is at this stage that it isimportant to be able to assess or measure the ripeness of the fruit sothat the importer or distributor may control the ripening to the degreenecessary for the fruit to be supplied to the retail outlet with therequired shelf-life.

One current method of testing fruit, such as avocado pears, toinvestigate the ripeness is to use a penetrometer. This is a hand-heldinstrument which comprises a pin or spike for pushing into the fruit,and a force meter which detects the force required to push the spikeinto the fruit and, hence, the degree of ripeness. Another instrumentdevised by the industry for testing the ripeness of an avocado pear is afirmometer. This instrument utilises a lever for applying a fixed forceto the exterior of the fruit and measures the resulting deflection ofthe lever to provide a reading indicative of ripeness. U.S. Pat. No.5,315,879 describes a measuring apparatus which may be used formeasuring the firmness of fruit and other objects and which operates onsimilar principles to a firmometer. Both the penetrometer and firmometertype of instrument have the disadvantage that they damage or bruise thefruit being tested so that, particularly, in the case of thepenetrometer, the fruit tested becomes unsaleable. Hence, they are usedfor testing on a selective basis and do not enable each individual fruitto be tested and individually assessed for shelf-life and treated and/orpackaged accordingly.

EP-A-0 267 737 describes apparatus for testing all fruit in a batch soas to measure individual ripeness. It makes use of a transducercomprising a polymeric piezoelectric film having electrodes and securedby adhesive to a metal plate which in turn is mounted on a resilientblock of foam material. The fruit to be tested is caused to impact onthe transducer which produces an electrical output from the film. Themetal plate is selected so as to have a mass which is small in relationto that of the fruit and is made of a metal which is non-resonant underthe impact. The foam support is such that the film, the plate and thefruit move in contact during the impact. This arrangement has the resultthat the output signal from the film represents the resonance of thefruit due to the impact, which can be used as a measure of the firmnessor ripeness of the fruit.

WO094/29715 describes a method and apparatus for testing the quality ofa fruit by applying a dynamic force to the fruit and detecting themechanical response of the fruit by means of piezoelectric filmtransducers supported on a displaceable supporting member. Hence, in sofar as it measures the mechanical response of the fruit, it is similarto EP-A-0 267 727.

EP-A-0 351 430 and U.S. Pat. No. 4,542,639 both relate to the impacttesting of engineering structures, such as laminates or honeycombconstructions, for delaminations and desponds. In the arrangement ofEP-A-0 351 430, a sensor is mounted on a hammer which imparts anon-destructive impact to a material which is to be measured, and asignal produced by striking the material with the hammer is used toindicate the period of time of contact of the hammer with the material.The impact drive force applied to the hammer is removed by using asignal from the sensor as soon as the hammer comes into contact with thematerial being measured. U.S. Pat. No. 4,542,639 describes the impacttesting of structures in which a structure is struck by an impactorassociated with a force transducer the output of which is related to theforce which the transducer experiences on impact and encompasses afrequency range including the lowest frequencies which that forcecontains to any substantial degree. The impactor may be driven by anelectromagnetic arrangement. The maximum amplitude indicates whether thestructure is faulty or not.

U.S. Pat. No. 4,217,614 describes apparatus for automatically applyingpressure sensitive labels to objects which utilises an applicationdevice in the form of a bellows which is pressure/vacuum operated inorder to extend and contract the bellows in a label applying operation.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of andapparatus for testing a fruit to assess or measure its ripeness and toprovide such a method and apparatus which are able to produce moreconsistent and reliable measurements of ripeness than hitherto knowninstruments and which produce such measurements without unacceptabledamage to the fruit being tested.

From one aspect, the present invention consists in a method of testing afruits to assess its ripeness, comprising the steps of applying adriving force to an impactor so as to cause the impactor to strike thefruit, detecting the reaction by means of a force transducerincorporated in the impactor and producing an electrical output signalrepresenting a reaction force generated by the impactor striking fruit,and processing the output signal to produce a measurement indicative ofthe ripeness of the fruit, characterised by terminating the applicationof the driving force before the impactor strikes the fruit so that theimpactor strikes the fruit with an impact in the form of a tap and thetransducer produces an electrical output signal in the form of a pulsein response to the reaction force generated by the impactor striking thefruit.

From another aspect, the invention consists in apparatus for testing afruit to assess its ripeness, comprising at least one impactor having aforce transducer which, when the impactor strikes the fruit, produces anelectrical output signal representing the reaction force generated bythe impact, driving means operable to apply a driving force to theimpactor and urge the impactor towards the fruit, and means forprocessing the output signal to produce a signal indicative of theripeness of the fruit, characterised in that the driving means isoperated so as to terminate the application of the driving force beforethe impactor strikes the fruit, whereby the impactor strikes the fruitwith an impact in the form of a tap and the transducer produces anoutput signal in the form of a pulse, said output signal being linearlyrelated to the reaction force to which the transducer is subjected byreason of the impact.

The force of the tap with which the fruit is struck must be of such amagnitude that it is not so small that the skin of the fruit absorbsmost of the blow and not so hard as to damage the fruit.

According to one preferred embodiment of the invention, the impactor isprovided in a plunger means which is adapted to move the impactortowards and away from a fruit item. Preferably, the plunger means is abellows which can be expanded by the admission thereto of pressurisedair and retracted by application of a vacuum. In this embodiment, theimpactor is movable relatively to the plunger means so that, when theplunger means stops moving towards a fruit item whose condition is to beassessed, the impactor will continue to move under its own momentum tostrike the surface of the fruit. By adjusting the speed of the plungermeans and the distance that the impactor travels, the force with whichthe impactor strikes the surface of the fruit is of the desiredmagnitude, as explained above.

When a fruit, such as an avocado pear, is tapped with the impactor, thereaction force resulting from the tap is detected by the forcetransducer and the latter produces an electrical output signal in theform of a single pulse corresponding to the reaction force. Both thepeak value and the duration of this pulse depend on the firmness and,therefore, the ripeness of the fruit. The peak value of the reactionforce and resulting electrical pulse increase as the firmness of thefruit increases whilst the duration of the pulse decreases with increasein firmness. The electrical pulse can be processed in several differentways in order to derive from the pulse an indication of the ripeness ofthe fruit tapped. Hence, the measurement of ripeness may be based onpeak force or the peak value of the resulting electrical output pulse.In order for such a measurement to be reliable, the momentum of theimpactor at the instant before striking the fruit must be constant forthe fruits being tapped. In practice, this may be difficult to achievewith irregularly shaped fruit. Alternatively, the output signal may beprocessed on the basis of duration in order to produce an indication ofripeness. The duration is only a weak function of the momentum of theimpactor on striking the fruit so that maintaining constant momentum atthis stage is not as important as when processing is based on peakvalue. However, problems may occur with the accuracy of measurementbased on duration because of the difficulty in accurately defining theduration of a pulse owing to the fact that there is frequently a “tail”on the pulse.

Instead of time domain measurements, as described above, the signalprocessing may involve some form of frequency domain processing. In oneform of the latter, the output signal is electronically resolved into afrequency spectrum encompassing a predetermined frequency range,including the lowest frequency which the output pulse comprises to anysignificant degree, and the frequency components in the frequencyspectrum are processed as a function of the reaction force. Preferably,such a processing stage comprises computing a graph of the variation ofthe frequency components in the frequency spectrum as a function of thereaction force based on a logarithmic scale (frequency along the x-axis,log force along the y-axis) and measuring the ripeness based on the areaof a predetermined zone below the graph and between, for example, twolines of constant force F1,F2 corresponding respectively to the logvalues of the maximum force component and a force component 25 dB lessthan the maximum. In order to provide a numerical output directlyrelated to the ripeness of each individual fruit of a particularspecies, the measured area of the graph may be presented as a percentageof a fixed reference area which, in the present example, may be selectedas the rectangular area defined between the lines of constant force F1,F2, and the same frequency limits as the measured area. The lower end ofthe frequency range may be substantially zero frequency and the upperend may be in the range from 2-5 kHz. The area calculated issubstantially independent of the level of the spectrum at zero frequencyand is therefore only a very weak function of the momentum of theimpactor at impact.

Another form of frequency domain processing is electronically to plot agraph of force against frequency on a linear force scale and simplyintegrate force with respect to frequency, thus obtaining the area underthe curve. This avoids the need to define predetermined down points asis required by the previously described frequency domain process. Thearea under the curve of the graph increases as the firmness increases.However, with this method of processing, the momentum of the impactormust be controlled very accurately as the area under the curve isproportional to momentum and, in practice, this method may not be a veryattractive.

One way of alleviating the effect of the momentum of the impactor atimpact on the momentum dependent measurements described above is tocompute the momentum H and normalise the result to produce a newmeasurement parameter given by peak force/H. Momentum H is given by theexpression: H = ∫P(t)t

where P(t) is the force as a function of time. This parameter works toprovide acceptable results but the peak force may not always be welldefined. An alternative, which uses all the points in the electricaloutput pulse representing the force-time function, is to compute theintegral of the square of the pulse S, which is given by the expression;S = ∫[P(t)]²t

Thus, normalising the above expression, the resulting parameter S/Hgives a more reliable measure of the firmness of the fruit.

The area under the force-frequency curve of the frequency domainprocessing described above may also be normalised by dividing bymomentum H although, in this case, a simpler normalisation is to divideby X(0) which is the dc (zero frequency) level of the spectrum which isobtained via the Fourier analysis utilised for converting fromforce-time to force-frequency.

The preferred method of signal processing is to use either the peakforce/H or the S/H parameter, as described above. This has the advantageof not requiring a Fourier transform and is quicker to implement thanfrequency domain techniques. It can also be implemented in analogueelectronics, rather than digital electronics, which makes the signalprocessing system potentially cheaper.

In order that the measurement can be provided as a numerical outputdirectly indicative of the ripeness of the fruit, it will be necessaryto calibrate the measurements produced against known ripening data foreach species of fruit and its individual cultivars.

The invention enables a ripeness test to be performed in any position ona fruit and the tap may be applied to the fruit either manually ormechanically. In an automated system having a mechanically operatedimpactor for tapping each individual fruit, in turn, to investigate theindividual ripeness of the fruit, the resulting signals indicative ofthe ripeness may be used, for example, to control a gating mechanismwhich directs the fruit to different collecting stations depending onthe degree of ripeness, and hence shelf-life, indicated by the ripenesssignal.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention may be more readily understood,reference will now be made to the accompanying drawings, in which:

FIG. 1 is a schematic side view of one embodiment of the invention,

FIG. 2 is a schematic side view illustrating the motion sequence of theimpactor device of FIG. 1 as it is engaged by fruit moving along aconveyor beneath the impactor device (for clarity the pivot position ofthe device is moved horizontally in this Figure whereas, in reality thepivot of the device is fixed and the fruit travels past the device),

FIG. 3 is a voltage/time graph illustrating the shapes of the electricaldriving pulse for the impactor and the output pulses resulting fromtapping fruits of different firmness,

FIG. 4 is a block circuit diagram of signal processing circuitrysuitable for use with the invention,

FIG. 5 is a diagrammatic, part sectional elevation, of anotherembodiment of the invention,

FIG. 6 is a section on an enlarged scale of the impactor of theembodiment of FIG. 5,

FIG. 6A is a fragmentary view of the impactor of FIG. 6, and

FIG. 7 is a view, partly in section, of apparatus embodying the impactordevice of FIGS. 5 and 6 and taken transverse to the fruit conveyor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The apparatus illustrated in FIG. 1 is designed to tap test fruit, suchas avocado pears, as they are conveyed along a so-called “singulator”which is used in sorting depots to place fruit into individual cups fromwhich they are deposited into different hoppers depending on the degreeof ripeness sensed by the test. The apparatus includes an impactor arm 1which is pivoted at one end 2 above the singulator or conveyor (notshown) arranged to convey the items of fruit one at a time beneath thearm. At its outer end, the arm mounts the impactor device 3. The lattercomprises a solenoid 4 having its armature 5 projecting at one end fromthe solenoid casing and serving as an impactor which is arranged to tapthe fruit passing beneath the arm. The armature 5 is advanced to apply atap to a fruit in response to an electrical driving pulse applied to thesolenoid and is spring biassed to return to its retracted position. Thearmature incorporates a force transducer in the form of a piezoelectriccrystal which produces an electrical output pulse in response to thereaction force exerted on the armature as a result of applying a tap toa fruit. The solenoid 4 is triggered to apply a tap in response to theactuation of a microswitch 6 by a fruit travelling beneath the impactorand engaging a downwardly projecting actuating arm 7 of the microswitch.

Between the solenoid 4 and the pivot 2, the arm 1 is fitted with rollers8 to permit the arm to ride smoothly over fruit travelling beneath andengaging the arm preparatory to being tapped by the impactor. The fruitis protected from damage by the outer end of the arm by a further roller9. Suitable stops 10, 11 are mounted below and above the arm adjacentits pivot in order to limit movement of the arm and prevent it fromdropping too low and engaging the conveyor or being raised too high.

The conveyor is of a known construction and, desirably, it shouldposition the avocado pears or other fruit, under the impactor with thewidest or most bulbous part of the fruit below the impactor. The fruitmay be advanced along the conveyor with a rolling motion or bestationary about its axis. Referring also to FIG. 2, as each fruit 12travels below the impactor arm 1, it engages the arm and pushes itupwards so as to move the impactor 3 into a position for tapping thefruit. When the fruit and impactor are in a predetermined positionrelative to one another, the fruit actuates the microswitch 6 byengaging the actuating arm 7 so that an electrical diving pulse issupplied to fire the solenoid 4 and the armature 5 is actuated to tapthe fruit.

The firing position of the solenoid is at A on large fruit 12 and at Bon small fruit 12′ whilst the first contact position is C on large fruitand D on small fruit. These differences in contact positions areaccommodated by firing the solenoid with the microswitch 6. Aftertapping, each fruit continues to travel beneath the arm 1 andsubsequently the arm is released from the fruit (position E) and returnsto a rest position against the lower stop preparatory to engaging thenext fruit on the conveyor line. The roller 9 at the outer end of thearm protects the fruit from damage as the arm is released.

As shown in the graph of FIG. 3, the solenoid driving pulse 13 is asquare pulse and has finished before the tap impacts on a fruit so thatthe solenoid 4 does not drive the armature into the fruit. The reactionforce resulting from a tap applied by the solenoid armature striking thefruit is detected by the force transducer and is reproduced as a singleelectrical output pulse similar to pulses 14, 15 shown in FIG. 3. Thepeak value and duration of the resulting output pulse depends on thefirmness and therefore the ripeness of the fruit. Hence, the pulse 14represents the pulse resulting from a tap test on an unripe or hardavocado whilst pulse 15 results from a tap test on a ripe or softavocado. These output pulses may be processed in any of the waysdescribed above in order to produce a measurement indicative of theripeness of the fruit.

FIG. 4 illustrates an electronic circuit for use with the tapping devicedescribed above and for processing the electrical output pulses producedby the device upon tapping fruit. The output pulses from thepiezoelectric transducer of the impactor device 3 are fed by way ofleads 16, an amplifier 17 and trigger unit 18 to an analogue-to-digitalconverter 19 and then to a buffer store 20. The trigger unit 18 operatesin response to actuation of the microswitch 6 and ensures that the valueof the output from the amplifier 17 covers the full duration of thepulse. When required, the output from the store 20 is fed to a computer21 which processes the digital signal from the store in any of the waysdescribed above to produce a measurement indicative of the ripeness ofthe fruit. In order that the measurement can be provided as a numericaloutput directly indicative of the ripeness, it will be necessary tocalibrate the measurements produced against known ripening data for eachspecies of fruit and its individual cultivars.

Referring now to FIGS. 5 and 6, an alternative embodiment of theimpactor device 28 comprises a bellows 30 of resilient material, suchas, plastics or synthetic rubber, and of lightweight construction. Sucha bellows is already known in connection with labelling machines forexample as described in U.S. Pat. No. 4,217,164. The bellows is mountedon the projecting annular flange 31 of a rigid, tubular support 32.Means (not shown) are provided for applying a vacuum to the bellows tohold it in a retracted disposition, as illustrated in FIG. 5, and whenappropriate, to supply pressurised air to the bellows to expand itdownwardly (as viewed in FIG. 5).

An impactor 33 is mounted on the inner surface of the free end 34 of thebellows above an aperture 35 in a shaped nose piece 36 at the free end34. The impactor 33 is movable with the bellows when the bellows isexpanded and retracted. It is electrically coupled by wires 37 to anamplifier 38 for signals from the impactor.

The impactor 33 is shown in more detail in FIG. 6. It is mounted in atubular housing 40 having an out-turned flange 41 at one end mountingthe impactor on the inner surface of the free end 34 of the bellows 30.A cap 42 is provided at the opposite end of the housing which with saidopposite end defines an Internal annular shoulder or abutment 43.

The impactor, itself, comprises an inner housing 44 slidably disposed inhousing 40. The end of the inner housing 44 adjacent the cap 42 isprovided with a flange 45. A compression spring 46 is positioned aroundthe inner housing and bears at one end on the shoulder 43 and at itsopposite end on the flange 45 so that the inner housing is urgedupwardly (as viewed in FIG. 6). The upward movement of the inner housingis limited by engagement of the inner housing against the cap 42.

Secured within the inner housing 44 is a solid slug 52 which mounts apiezoelectric transducer 50 adjacent the end 51 of the inner housingremote from the cap 42. The end 53 of the slug projects from the end 51of the inner housing for striking a fruit to be tested and as partspherically shaped. The transducer 50 is mounted in contact with theslug and the signal wires 37 are fed to a cavity 54 providing access toopposite sides of the transducer and permitting connection of the wires37 thereto, via an aperture 55 in the cap and passageways 56,57 in theinner housing and slug (see also FIG. 6A).

In operation fruit or vegetable items are conveyed in sequence by aconveyor past the bellows. When a fruit item is underneath the bellows,expansion of the bellows is effected in response to control means whichcan be similar to the control means used for labelling, as described inthe aforementioned U.S. Pat. No. 4,217,164. The bellows expand until thenose piece 36 at the free end contacts the fruit or vegetable item. Atthat instant further expansion of the bellows stops. However, theimpactor 33 which moves with the expanding bellows continues movinguntil the slug 52 impacts against the surface of the fruit or vegetableitem. The reaction force exerted on the slug 52 causes the piezoelectrictransducer 50 in contact with the slug to produce a signal which canthen be processed in the same way as described in connection with FIG.4.

In FIG. 7, the impactor device 28 is shown installed in a ripenesstesting machine and mounted above a fruit 60 which is passing below thedevice. The latter is mounted on a frame structure 58 which is disposedabove a conveyor 59 upon which the fruit 60 is transported.

The tubular support 32 for the bellows of the impactor devicecommunicates with a chamber 61 mounted on the frame structure 58 abovethe device. The chamber 61 is connected at one side, via a port 62, to apressurised air chamber 63 which is coupled to a source of air pressureby an inlet conduit 64. At its opposite side, the chamber 61 isconnected, via a port 65, to a vacuum chamber 66 which is connected to asource of vacuum by an outlet conduit 67. The ports 62,65 are controlledby valve members 68,69 attached to a slidable valve rod 70 which isreciprocated by means of an electrical solenoid 71 and a return spring72. The spring 72 urges the valve members 68,69 into positions in whichthe air inlet port 62 is closed and the vacuum port 65 is open so thatvacuum is applied to the support tube 32 and the bellows 30 are retainedin a retracted rest position. Actuation of the solenoid 71 slides thevalve control rod 70 against the action of the spring 72 to open the airinlet port 62 and close the vacuum port 65, thereby momentarilyexpanding the bellows so as to cause the nose 36 to contact a fruit 60conveyed below the impactor device and the impactor to tap the fruit andproduce an output pulse from the transducer 50. The solenoid 71 can becontrolled in any convenient manner so as to actuate the impactor deviceas each fruit 60 is advanced below it. The solenoid is triggered so asto open the valve member 68 only briefly and apply air pressure to thebellows for a sufficient time to produce a driving force to initiatemovement of the bellows and impactor towards the fruit, the arrangementbeing such that the impactor striking the fruit under its own momentumwhen the nose piece 36 of the bellows contacts and stops against thefruit. Immediately, thereafter, the bellows are contracted by exhaustionof air therefrom through the vacuum port 65 and vacuum outlet conduit 67to return the impactor device to its rest position.

In order to optimise the ripeness measurement for each fruit, two ormore impactor devices 28 may be mounted side-by-side in a rowtransversely of the conveyor 59 for simultaneously tapping each fruit soas to produce an output signal for each of a plurality of positionsalong the fruit axis disposed transversely to the direction of movementof the conveyor. The conveyor 59 may be adapted to rotate each fruit asit is advanced by the conveyor and a plurality of the impactor devices28 may also be mounted in succession, or in successive rows, along theconveyor for successively tapping each fruit and producing an outputsignal for each of a plurality of positions about the fruit.

Whilst particular embodiments have been described, it will be understoodthat modifications can be made without departing from the scope of theinvention as defined by the appended claims. For example, the signalprocessing may not require that the analogue output signal from thepiezoelectric transducer be converted into a digital signal forprocessing by the computer, in which event, the analogue to digitalconverter 19 may be omitted from the circuit. Moreover, the rollers 8,9on the impactor arm 1 many be replaced by strips of low frictionmaterial, such as PTFE.

What is claimed is:
 1. A method of testing a fruit to assess itsripeness, said method comprising the steps of: providing an impactor forstriking said fruit, said impactor incorporating a force transducer fordetecting a reaction force generated by said impactor striking saidfruit and producing an electrical output signal representing saidreaction force; applying a driving force to said impactor so as to causesaid impactor to strike said fruit; terminating application of saiddriving force before said impactor strikes said fruit so that saidimpactor strikes said fruit with an impact in the form of a tap, saidtransducer thereby producing an electrical output signal in the form ofa pulse in response to said reaction force generated by said impactorstriking said fruit, and processing said output signal to produce ameasurement indicative of the ripeness of said fruit.
 2. A methodaccording to claim 1, wherein the processing of the output signalinvolves determining the peak value of the reaction force.
 3. A methodaccording to claim 1, wherein the processing of the output signalinvolves resolving the output signal into a frequency spectrumencompassing a predetermined frequency range and processing thefrequency components of the spectrum as a function of the reactionforce.
 4. A method according to claim 3, including computing a graph ofthe variation of the frequency components in the frequency spectrum as afunction of the reaction force based on a logarithmic scale andproducing the measurement of ripeness based on the area of apredetermined zone below the graph.
 5. A method according to claim 3,wherein the processing of the output signal involves integrating forcewith respect to frequency for a plot of force against frequency on alinear force scale.
 6. A method according to claims 1, wherein theprocessing of the output signal involves computing the parameter S whichis given by the equation S = ∫[P(t)]²t

where P(t) is the reaction force as a function of time.
 7. A methodaccording to claim 1, wherein the effect of the momentum of theimpactor, at impact, on the ripeness measurement is alleviated bynormalising the measurement.
 8. A method according to claim 7, whereinthe normalisation involves dividing the measurement by the momentum H ofthe impactor at impact which is given by the expression H = ∫P(t)t

where P(t) is the reaction force as a function of time.
 9. A methodaccording to claim 4, wherein the frequency spectrum has a DC level andthe value obtained for said area below said graph is normalised bydividing by the DC level of the spectrum.
 10. A method according toclaim 1, wherein the fruit is rotated and is struck by a plurality ofthe impactors so that output pulses are produced for a plurality ofpositions about the fruit.
 11. Apparatus for testing a fruit to assessits ripeness, said apparatus comprising: at least one impactor, saidimpactor having a force transducer which, when said impactor strikessaid fruit produces an electrical output signal representing a reactionforce generated by said impactor striking said fruit; driving means forapplying a driving force to said impactor to urge said impactor towardssaid fruit, said driving means being operable so as to terminateapplication of said driving force before said impactor strikes saidfruit, whereby said impactor strikes said fruit with an impact in theform of a tap and said transducer produces an output signal in the formof a pulse, said output signal being linearly related to said reactionforce to which said transducer is subjected by reason of said impact;and means for processing said output signal to produce a signalindicative of the ripeness of said fruit.
 12. Apparatus according toclaim 11, wherein the impactor is mounted in a plunger means which isadapted to move the impactor towards and away from the fruit. 13.Apparatus according to claim 12, wherein the plunger means is a bellowswhich is arranged to be expanded by the admission of a pressurised gasand retracted by the application of vacuum.
 14. Apparatus according toclaim 12, wherein the impactor is movably mounted relatively to theplunger such that, when the plunger stops moving towards the fruit, theimpactor continues to move under its own momentum so as to strike thefruit.
 15. Apparatus according to claim 11, wherein the impactorcomprises the armature (4) of an electrical solenoid (4) serving as thedriving means.
 16. Apparatus according to claim 11, wherein the impactor(1) is mounted at the end of a pivoted arm (1) which is engageable bythe fruit advanced beneath the arm to position the impactor for strikingthe fruit.
 17. Apparatus according to claim 16, wherein the arm mounts aplurality of rollers, PTFE strips or other friction reducing means inpositions to engage the fruit advanced beneath the arm to permit the armto ride smoothly over the fruit.
 18. Apparatus according to claim 11,wherein the transducer comprises a piezoelectric crystal.
 19. Apparatusaccording to claim 11, including conveying means for advancing the fruitrelatively to the impactor.
 20. Apparatus according to claim 19, whereinthe impactor is triggered by the fruit contacting a microswitch as it isadvanced by the conveying means relatively to the impactors. 21.Apparatus according to claim 19, including at least two of the impactorsmounted side-by-side transversely to the conveying means so as to tapthe fruit and produce an output signal for each of a plurality ofpositions along an axis of the fruit which axis is disposed transverseto the direction of movement of the conveying means.
 22. Apparatusaccording to claim 19, wherein the conveying means is adapted to rotatethe fruit as it is advanced by the conveying means, and a plurality ofthe impactors are mounted in succession along the conveying means so asto tap the fruit and produce an output signal for each of a plurality ofpositions about the fruit.
 23. Apparatus for testing a fruit to assessits ripeness, said apparatus comprising: at least one impactor, saidimpactor having a force transducer which, when said impactor strikessaid fruit produces an electrical output signal representing a reactionforce generated by said impactor striking said fruit; a bellows mountingsaid impactor; control means for alternately admitting a pressurised gasand applying a vacuum to said bellows, whereby to expand said bellowsand apply a driving force to said impactor for urging said impactortowards said fruit, and thereafter retract said bellows; said controlmeans being arranged to terminate said application of pressurised gasbefore said impactor strikes said fruit, whereby said impactor strikessaid fruit with an impact in the form of a tap and said transducerproduces an output signal in the form of a pulse, said output signalbeing linearly related to said reaction force to which said transduceris subjected by reason of said impact; and means for processing saidoutput signal to produce a signal indicative of the ripeness of saidfruit.
 24. Apparatus for testing a fruit to assess its ripeness, saidapparatus comprising: a plurality of impactors, each said impactorhaving a force transducer which, when said impactor strikes said fruit,produces an electrical output signal representing a reaction forcegenerated by said impactor striking said fruit; conveying means foradvancing said fruit relatively to said impactors and for rotating saidfruit as it is advanced, said impactors being mounted in successionalong said conveying means so as to strike said fruit and produce saidoutput signals for a plurality of positions about said fruit; drivingmeans for applying a driving force to each said impactor to urge saidimpactor towards said fruit, said driving means being operable so as toterminate application of said driving force to said impactor before saidimpactor strikes said fruit, whereby said impactor strikes said fruitwith an impact in the form of a tap and the respective transducerproduces an output signal in the form of a pulse, said output signalbeing linearly related to said reaction force to which said transduceris subjected by reason of said impact; and means for processing eachsaid output signal to produce a signal indicative of the ripeness ofsaid fruit.